In electrophotography, an image having a pattern of electrostatic potential, also referred to as an electrostatic latent image, is formed on a surface of an electrophotographic element comprising at least two layers: a photoconductive layer and an electrically conductive substrate. The electrostatic latent image can be formed by a variety of methods, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
Among the many different kinds of photoconductive materials that have been employed in electrophotographic elements are phthalocyanine pigments such as titanyl phthalocyanine and titanyl tetrafluorophthalocyanines. Electrophotographic recording elements containing such pigments as charge-generation materials are useful in electrophotographic laser beam printers because of their capability for providing good photosensitivity in the near infrared region of the electromagnetic spectrum, that is, in the range of 700–900 nm.
Flocculation of organic pigment dispersions has been a recognized problem, especially in the paint and coating industry, for some time. For example, U.S. Pat. No. 3,589,924, in the names of Giambalvo, et al., describes improved non-crystallizing, non-flocculating phthalocyanine pigment compositions formed by mixing 60–95% of a crystallization-or flocculation-susceptible phthalocyanine pigment with about 5–40% of a sulfonated phthalimidomethyl phthalocyanine derivative. The mixture is prepared by the usual methods, e.g., acid pasting or salt grinding, for converting the phthalocyanine materials to pigmentary size. However techniques that are designed primarily to provide suitable pigments for paints and industrial coatings may not yield materials of sufficient purity or the appropriate crystallinity characteristics to meet the stringent requirements of electrophotographic applications, where high purity is very important for ensuring reliable performance. The crystalline form of the final pigment also has a profound influence on the performance of an electrophotographic element containing it.
In a photoconductive layer produced from a liquid coating composition that includes the titanyl phthalocyanine pigment and a solvent solution of polymeric binder, it is necessary that the titanyl phthalocyanine pigment be in a highly photoconductive form, either crystalline or amorphous, and in a sufficiently stable dispersion to permit its application as a very thin layer having high electrophotographic speed in the near infrared region.
A variety of methods have been used to produce suitable forms of titanyl phthalocyanine having differing crystallographic characteristics. U.S. Pat. No. 5,166,339 in the names of Duff, et al., presents a table of polymorphs of unsubstituted titanyl phthalocyanine in which materials bearing multiple designations are grouped as four types. Many phthalocyanine pigments are discussed in P. M. Borsenberger and D. S. Weiss, Organic Photoreceptors for Imaging Systems, Marcel Dekker, Inc., New York, pp. 338–391.
In one type of preparation, commonly referred to as “acid-pasting”, crude titanyl phthalocyanine is dissolved in an acid solution, which is then diluted with a non-solvent to precipitate the titanyl phthalocyanine product. In another type of procedure, the crude titanyl phthalocyanine is milled, generally with particular milling media. Additionally, some preparations include a combination of techniques or modify a previously prepared titanyl phthalocyanine.
U.S. Pat. No. 5,132,197 in the names of Iuchi, et al., teaches a method in which titanyl phthalocyanine is acid pasted, treated with methanol, and milled with ether, monoterpene hydrocarbon, or liquid paraffin to produce a titanyl phthalocyanine having main peaks of the Bragg angle 2Θ with respect to X-rays of Cu Kα at 9.0°, 14.2°, 23.9°, and 27.1° (all +/−0.2°).
U.S. Pat. No. 5,206,359 in the names of Mayo, et al., teaches a process in which titanyl phthalocyanine produced by acid pasting is converted to type IV titanyl phthalocyanine from Type X by treatment with halobenzene.
U.S. Pat. No. 5,059,355 in the names of Ono, et al., teaches a process in which titanyl phthalocyanine is shaken with glass beads, producing an amorphous material having no substantial peaks detectable by X-ray diffraction. The amorphous material is stirred, with heating, in water and ortho-dichlorobenzene; methanol is added after cooling. A crystalline material having a distinct peak at 27.3° is produced.
U.S. Pat. No. 4,882,427 in the names of Enokida, et al., teaches a material having noncrystalline titanyl phthalocyanine and pseudo-non-crystalline titanyl phthalocyanine. The pseudo-noncrystalline material can be prepared by acid pasting or acid slurrying. The noncrystalline titanyl phthalocyanine can be prepared by acid pasting or acid slurrying followed by dry or wet milling, or by mechanical milling for a long time without chemical treatment.
U.S. Pat. No. 5,194,354 in the names of Takai, et al., teaches that amorphous titanyl phthalocyanine prepared by dry pulverization or acid pasting can be converted, by stirring in methanol, to a low crystalline titanyl phthalocyanine having strong peaks of the Bragg angle 2Θ with respect to X-rays of Cu Kα at 7.2°, 14.2°, 24.0°, and 27.2° (all +/−0.2°). It is stated that the low crystalline material can be treated with various organic solvents to produce crystalline materials: methyl cellosolve or ethylene glycol for material having strong peaks at 7.4°, 10.9°, and 17.9°; propylene glycol, 1,3-butanediol, or glycerine for material having strong peaks at 7.6°, 9.7°, 12.7°, 16.2°, and 26.4°; and aqueous mannitol solution for material having strong peaks at 8.5° and 10.2° (all peaks +/−0.2°).
U.S. Pat. Nos. 4,994,566 and 5,008,173 both in the names of Mimura, et al., teach a process in which non-crystalline particles produced by acid pasting or slurrying, followed by mechanical grinding or sublimation, are treated with tetrahydrofuran to produce a titanyl phthalocyanine having infrared absorption peaks at 1332, 1074, 962, and 783 cm−1.
U.S. Pat. No. 5,039,586 in the name of Itami, teaches acid pasting, followed by milling in aromatic or haloaromatic solvent, with or without additional water or other solvents such as alcohols or ethers, at 20–100° C. In an example, crude titanyl phthalocyanine is milled with α-chloronaphthalene or ortho-dichlorobenzene as milling medium, followed by washing with acetone and methanol. The titanyl phthalocyanine produced has a first maximum intensity peak of the Bragg angle 2Θ with respect to X-rays of Cu Kα at a wavelength of 1.541 Å at 27.3°+/−0.2°, and a second maximum intensity peak at 6.8°+/−0.2°. This was contrasted with another titanyl phthalocyanine that is similarly milled, but not acid pasted. This material has a maximum peak at 27.3°+/−0.2° and a second maximum intensity peak at 7.5°+/−0.2°.
U.S. Pat. No. 5,055,368, in the names of Nguyen, et al., teaches a “salt-milling” procedure in which crude titanyl phthalocyanine is milled, first under moderate shearing conditions with milling media comprising inorganic salt and non-conducting particles. The milling is then continued at higher shear and temperatures up to 50° C., until the pigment undergoes a perceptible color change. Solvent is substantially absent during the milling steps.
U.S. Pat. No. 4,701,396 in the names of Hung, et al., teaches near infrared sensitive photoconductive elements made from fluorine-substituted titanylphthalocyanine pigments. While phthalocyanines having only fluorine substituents, and those being equal in number on each aromatic ring, are the preferred pigments of the invention described in that patent, various non-uniformly substituted phthalocyanines are taught.
U.S. Pat. No. 5,112,711 in the names of Nguyen, et al., teaches an electrophotographic element having a physical mixture of titanyl phthalocyanine crystals and titanyl fluorophthalocyanine crystals. The element provides a synergistic increase in photosensitivity in comparison to an expected additive combination of titanyl phthalocyanine and titanyl fluorophthalocyanine. Similar elements having physical mixtures combining titanyl phthalocyanine and chloro- or bromo-substituted titanyl phthalocyanine crystals produce results in which the photosensitivity is close to that of the least sensitive of the two phthalocyanines used.
U.S. Pat. Nos. 5,238,764 and 5,238,766, both in the names of Molaire, et al., teach that titanyl fluorophthalocyanine products of acid-pasting and salt-milling procedures, unlike unsubstituted titanyl phthalocyanine, suffer a significant reduction in near infrared sensitivity when they are dispersed in a solvent such as methanol or tetrahydrofuran, which has a gammac hydrogen bonding parameter value greater than 9.0. These patents further teach that this reduction in sensitivity can be prevented by first contacting the titanyl fluorophthalocyanine with a material having a gammac hydrogen bonding parameter of less than 8.0.
Molaire, et al., in U.S. Pat. No. 5,629,418, discloses a method for preparing titanyl fluorophthalocyanine that includes the steps of: dissolving titanyl fluorophthalocyanine in acid to form a solution; admixing the solution and water to precipitate out amorphous titanyl fluorophthalocyanine; washing the amorphous titanyl fluorophthalocyanine until substantially all of the acid is removed and contacting it with an organic solvent, which results in the conversion of the amorphous material to high crystallinity titanyl fluorophthalocyanine, the amorphous titanyl fluorophthalocyanine having been maintained in contact with water continuously from its precipitation to its conversion to a crystalline form.
In U.S. Pat. Nos. 5,614,342 and 5,766,810 both in the names of Molaire and Kaeding, disclose a method for preparing cocrystals of titanyl fluorophthalocyanine and unsubstituted titanyl phthalocyanine that includes the steps of: admixing crude titanyl phthalocyanine and crude titanyl fluorophthalocyanine to provide an amorphous pigment mixture, as determined by X-ray crystallography using X-radiation characteristic of Cu Kα at a wavelength of 1.541 Å of the Bragg angle 2Θ; contacting the amorphous pigment mixture with an organic solvent having a gammac hydrogen bonding parameter of less than 8:0; and, prior to contacting, substantially excluding the amorphous pigment mixture from contact with an organic solvent having a gammac hydrogen bonding parameter greater than 9.0. The amorphization step must be substantially complete so as to break the large primary particles of the starting crude pigments and thereby lower the average particle size of the final cocrystalline mixture. Substantially complete amorphization of the crude pigments is also necessary to prevent degradation of the dark decay characteristics of the final cocrystal; small amounts of crude pigments having inherently high dark decay that are not amorphized would not be affected by the subsequent solvent treatment and therefore would retain their high dark decay characteristics, causing degradation of the dark decay property of the final cocrystalline product.
Molaire, et al., in U.S. Pat. No. 5,523,189, discloses an electrophotographic element including a charge generation layer that includes a binder in which is dispersed a physical mixture of a high speed titanyl fluorophthalocyanine having a first intensity peak with respect to X-rays characteristic of Cu Kα at a wavelength of 1.541 Å of the Bragg angle 2Θ at 27°±0.2°, and a second intensity peak at 7.3°±0.2°, the second peak having an intensity relative to the first peak of less than 60 percent; and a low speed titanyl fluorophthalocyanine having a first intensity peak with respect to X-rays characteristic of Cu Kα at a wavelength of 1.541 Å of the Bragg angle 2Θ at 6.7°±0.2°, and a second intensity peak at 23°±0.2°, the second peak having an intensity relative to the first peak of less than 50 percent.
Molaire, et al., in U.S. Pat. No. 5,773,181, discloses a method for preparing a phthalocyanine composition including the steps of: synthesizing a crystalline product comprising a mixture of five different unsubstituted or fluorosubstituted phthalocyanines, wherein a central M moiety bonded to the four inner nitrogen atoms of the phthalocyanine nuclei represents a pair of hydrogen atoms or a covalent or coordinate bonded moiety, including an atom selected from the group consisting of Li, Na, K, Be, Mg, Ca, Ba, Sc, Y, La, Ac, Ti, Zr, Hf, V, Nb, Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Al, Ga, In, Tl, Si, Ge, Sn, Pb, As, and Sb, with M preferably representing Ti═O; increasing the amorphous character of the mixture of phthalocyanines as determined by X-ray crystallography using X-radiation characteristic of Cu Kα at a wavelength of 1.541 Å of the Bragg angle 2Θ to provide an amorphous pigment mixture; contacting the amorphous pigment mixture with organic solvent having a gammac hydrogen bonding parameter of less than 8.0; and prior to the contacting, substantially excluding the amorphous pigment mixture from contact with organic solvent having a gammac hydrogen bonding parameter greater than 9.0.
The procedures for the preparation of titanyl phthalocyanine pigments described in the foregoing patents, all of whose disclosures are incorporated herein by reference, suffer from various deficiencies and disadvantages. For example, the use of acid presents a shortcoming for both environmental and safety concerns, particularly in commercial scale procedures. Also, salt milling avoids the use of acid but requires extensive washing of the milled material to remove salts, which can otherwise cause high dark decay in a photoconductor.
Procedures that first contact the titanyl fluorophthalocyanine with a solvent such as methanol or tetrahydrofuran that has a gammac hydrogen bonding parameter value greater than 9.0 cause a significant reduction in near infrared sensitivity. The preparation of titanyl fluorophthalocyanine having good photogeneration characteristics is expensive. It would be desirable to be able to produce a crystalline titanyl phthalocyanine composition that has good photogeneration characteristics when used in an electrophotographic element but is less expensive than titanyl fluorophthalocyanine. A suitable procedure would avoid deleterious contact with high gammac hydrogen bonding parameter solvents and also not require the use of acid or of salt milling media.
Because of environmental concerns regarding the use of certain solvents such as chlorinated hydrocarbons, i.e., dichloromethane and trichloroethane, it would be desirable to form photoconductive layers from coating compositions containing nonchlorinated solvents such as acetone, tetrahydrofuran (THF), methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cyclohexanone, and alcohols, including methanol, ethanol, isopropanol, butanol, and 2-ethoxyethanol. The present inventors have found, however, that the cocrystalline mixtures described in the above-discussed U.S. Pat. Nos. 5,614,342 and 5,766,810 are difficult to disperse in non-chlorinated solvents and have further found that the dispersions using the non-chlorinated solvents tend to be unstable toward flocculation.
After considerable investigation and experimentation, the present inventors have discovered a process for producing cocrystals of titanyl fluorophthalocyanines, abbreviated herein as “TiFOPc”, and unsubstituted titanyl phthalocyanine, abbreviated herein as “TiOPc”, that can be monodispersed into nanoparticles without the use of chlorinated solvents or of binder materials. Furthermore, the inventors have found that electrophotographic performance such as sensitivity can be improved by the process of their invention.